Field of the Invention
The present invention relates to a sliding member, and specifically relates to a sliding member including a back metal layer and a sliding layer including synthetic resin and graphite.
Description of the Related Art
A sliding member has been conventionally used, which includes a resin composition added with graphite flakes, as a solid lubricant, in a synthetic resin matrix (see JP-A-2005-089514). In general, natural graphite is classified to flake graphite, vein graphite, and amorphous graphite according to their properties. The vein graphite has the highest graphitization degree of 100%. The flake graphite has the graphitization degree of 99.9%, and the amorphous graphite has the low graphitization degree of 28%. Conventionally, the natural graphite of the vein or flake graphite having the high graphitization degree has been mechanically grinded to produce flake-like particles to be used as a solid lubricant of a sliding member.
The flake-like shaped graphite has a crystal structure including a large number of laminated AB planes (hexagonal net planes, or basal surfaces) where carbon atoms form a net structure regularly and extend in a planar shape, and defining a thickness in C-axis direction which is perpendicular to the AB plane. A bonding strength, by van der Waals force, is much weak between the laminated AB planes compared with an in-plane direction of the AB plane. Thus, shearing easily occurs between the AB planes. Accordingly, this graphite has a smaller thickness of the lamination in comparison with the extension of the AB plane, and thus the graphite generally has a thin plate shape. The flake-like graphite particles function as a solid lubricant since the shearing occurs between the AB planes when an external force is applied thereto.
In recent years, there is a problem that flake-like graphite particles break and drop off from a sliding member having a resin composition including the flake-like graphite particles, when a surface of the resin composition, which forms a sliding surface, is subjected to machining, since the flake-like graphite particles have a thin plate shape and are brittle. In the case, the sliding layer has a larger surface roughness, resulting in deterioration of seizure resistance. In order to solve the problem, WO 2012/074107, for example, proposes a sliding material including spherical natural graphite particles in a synthetic resin to improve a surface roughness after the machining.
The spherical graphite particles are produced by repeatedly applying a small load on the natural flake graphite particles as a raw material for bending them, and thereby granulating them to have a spherical shape (see WO 2012/137770 or JP-A-2008-024588).